The present invention relates to a centerless grinder and a wheel truing device for the centerless grinder, and in particular, to a centerless grinder and a wheel truing device for use in grinding a stem and a head simultaneously of a workpiece such as an engine valve.
Engine valves are finished by separate mechanical processes in which a stem is processed with a centerless grinder and a valve face of a head is ground with a grinder that uses a chuck. However, when the stem and the valve face are ground by different grinders respectively, process productivity is poor and the number of machines required is increased. Furthermore, errors are likely to be caused in the concentricity and size accuracy between the stem and the valve face.
To overcome these disadvantages, it is suggested that the stem and the valve face are ground simultaneously using a centerless grinder as shown in FIG. 6.
To a rotary shaft 102 of a motor 101, a first grinding wheel 104 for grinding a stem 103a of an engine valve 103 and a second grinding wheel 105 for grinding a valve face 103c of a head 103b are mounted. Between the first grinding wheel 104 and an adjusting wheel 107 rotated at lower speed by a motor 106, the stem 103a of the engine valve 103 is held, and the stem 103a and the valve face 103c are simultaneously ground by the first and second grinding wheels 104 and 105 respectively by rotating the motors 101 an 106 in the same direction.
However, in the centerless grinder in which the two grinding wheels 104 and 105 are mounted to a single shaft, the rotation speeds of the grinding wheels 104 and 105 are the same, so that the stem 103a and the valve face 103c are ground at different processing speeds owing to the difference in circumferential speed which depends on the difference in outer diameters of the wheels. That is to say, if the circumferential speed of the first grinding wheel 104 for grinding the stem 103a is set to the most suitable grinding condition, the circumferential speed of the second grinding wheel 105 for grinding the valve face 103c becomes too slow, thereby increasing the surface roughness of the valve face 103c, which requires a good surface finish, so that further finish polishing is required. Conversely, if the circumferential speed of the second grinding wheel 105 is set to the most suitable grinding condition, then the circumferential speed of the first grinding wheel 104 is too fast, thereby causing burning of the stem 103 and rapid wearing of the first grinding wheel 104.
The first and second grinding wheels 104 and 105 are mounted on the same shaft in the conventional grinder above, and as their respective removal amounts are not separately controlled, the grinding wheels 104 and 105 must be trued depending on the extent of wear. Also, when the size of the workpiece and the inclined angle of the valve face are varied, then, the second grinding wheel 105 must be replaced with one of suitable dimensions, thereby requiring complicated setting-up and preparation with a resultant decrease in productivity.
To true the first and second grinding wheels, if they are to be trued by a single dresser, will take a lot of time, thereby further decreasing efficiency and productivity. If the two wheels have different circumferential surfaces (grinding surfaces) and are trued using one dresser, the truing surface of the dresser itself does not wear uniformly, requiring its early replacement. Further, even if the grinding wheels contain different grinding particles (are of different hardness) and have a different speed of wear, they are simultaneously trued at fixed intervals depending on the number of workpieces ground. Therefore, if the time for truing one grinding wheel is set to the most suitable condition, the other wheel will be either excessively or insufficiently trued.
An object of the present invention is to provide a centerless grinder in which the most suitable grinding conditions are available depending on the dimensions or grinding portion sizes of a workpiece to decrease the number of times replacement of a grinding wheel and to facilitate operation of the grinder.
Another object of the present invention is to provide a wheel truing device for a centerless grinder in which grinding wheels are rapidly trued to the most suitable grinding conditions even if the grinding wheels are of different types and have different wear speeds.